Although the present invention can be applied to a large number of electrical and electronic systems, in the following said invention is described in relation to a photovoltaic system comprising a solar inverter.
Photovoltaic systems are used to obtain electrical energy from solar energy and to feed this electrical energy into a public power supply network or power grid. For this purpose, inverters are customarily used which convert the electrical direct current generated by the solar cells of the photovoltaic system into an alternating current. Said alternating current can then be fed into the utility grid.
In order to be able to feed an alternating current that is as symmetrical as possible into the utility grids, the solar cells or the strings of solar cells are for example directly coupled to the inverter. In doing so, the negative pole of the solar cell arrangement is coupled to the negative pole of the inverter and the positive pole of the solar cell arrangement is coupled to the positive pole of the inverter. The problem with connecting solar cells and inverters in this way is that in some circumstances, in the event of low solar insolation, the DC voltage generated by the solar cells turns out to be too low to generate an alternating current, by means of the inverter, that can then be fed into the utility grid. In inverter topologies having a split intermediate circuit (UZK+ and UZK−), the size of the voltage in the negative and positive intermediate circuit has to be greater than the amplitude of the mains voltage. With a 230 V mains voltage, the amplitude is 324 V. If the solar cell arrangement of a photovoltaic system generates for example a DC voltage of less than 2×324 V=648 V, the DC voltage thus obtained can no longer be converted into a 230 V AC voltage by an inverter having a split intermediate circuit.
For this reason, what are known as boost converters are used between the solar cell arrangement and the inverter and increase the amount of DC voltage generated by the solar cell arrangement. As a result, it is possible to generate and feed alternating current for a utility grid even when the solar cell arrangement supplies a lower amount of DC voltage than is required for the utility grid.
If the solar cell arrangement is connected to the inverter such that the positive pole of the solar cell arrangement has a positive potential with respect to the reference ground and the negative pole of the solar cell arrangement has a negative potential with respect to the reference ground, this can lead to a creeping performance degradation of the solar cells as a result of what is known as the PID effect. This PID effect is caused substantially by the negative potential of the negative pole of the solar cell with respect to the potential of the reference ground, which can lead to undesired leakage currents. Overall, this leads to accelerated aging and a significant performance loss of the solar cell arrangement.
In order to prevent this PID effect from occurring, electrical circuits are used which are intended to raise the voltage potential of the negative pole of the solar cell, so that said potential is equal to the potential of the reference ground or takes on a positive value with respect to the potential of the reference ground.
A corresponding circuit system is described for example in DE 10 2007 050 554 A1. If the potential of the positive pole of the solar cell is increased, the potential of the negative pole of the solar cell increases too.